An image forming apparatus comprising a laser beam radiating unit utilized as a light source, and an optical system that the surface of an original reflects a laser beam radiated from the radiating unit to project an image reflected form the original onto a photosensitive member. In this apparatus, an image is formed selectively in a print mode that an image results from turning on and off the laser beam, in a copy made that a copy of an original results form exposure of the photosensitive member to the laser beam reflected from the surface of an original or in a composite mode that a composite image is formed of a copy of an original and an image resulting from turning on and off the laser beam. Also, this apparatus comprises a reflecting member which is provided on an original supporting table and extends in the sub scanning direction, and a sensor for detecting the laser beam reflected from the reflecting member. A signal produced by the sensor is used as a signal for setting a starting point in outputting image data, a start signal for starting outputting image data in the sub scanning direction and a completion signal for finishing the scanning.